1. Field of the Invention
This invention relates generally to fluid purification systems, and more particularly to methods of regenerating purifier materials used in process fluid purification systems.
2. Description of the Prior Art
The provision of high purity gas streams is critically important in a wide variety of industrial and research applications. The rapid expansion of vapor-phase processing techniques, e.g. chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment that is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility.
Fluid purification systems are used to remove impurities from contaminated or otherwise impure fluids for the production of high purity process gases. The fluids may be liquid and/or gaseous, and are typically purified to provide a high quality fluid source for manufacturing or consumption purposes, or to remove toxins and other contaminants prior to the disposal of the fluid. Such systems usually include “consumables” such as filters or getters, which must be periodically replaced. For example, ammonia is a process gas that is very important in the semiconductor industry for the formation of nitride layers in electronic transistors through chemical vapor deposition and epitaxy processes. More specifically, ammonia is commonly used for the formation of silicon nitride and silicon oxynitride films by direct nitridation of silicon oxide. Growing films of silicon nitride and silicon oxynitride requires ammonia of very high purity. Ammonia is also used in the production of compound semiconductors such as GaN, AlGaN, GaInN, etc. Oxygen is a particularly harmful contaminant because its high chemical reactivity leads to its incorporation as an impurity into films during thermal nitridation of silicon oxide or during the production of GaN and GaAlN semiconductors. Since the manufacturing of GaN films requires very large amounts of ammonia during the processing steps, new methods of ammonia purification are required to reduce the cost of manufacturing wafers in the GaN process. Regenerable purifiers are also becoming more important as the cost of ownership becomes a more critical parameter.
Because gas purifier materials tend to be expensive, semiconductor manufacturers want to get as much use out of them as possible before they are replaced. However, semiconductor manufacturers tend to err on the side of conservatism, since using a gas purifier material past its allotted lifetime can result in inadequate gas purification and a possible contamination of a semiconductor manufacturing process utilizing the impure gas, which can be considerably more costly than the cost of replacing a gas purifier vessel.
Gas purifiers working on sorption of impurities mainly belong to two categories: getter-based purifiers and catalyst-based purifiers. Getter-based purifiers operate using the chemisorption principle, that is, non-reversible sorption, so that once the purifier is exhausted it must be replaced. In these systems, getter materials are encased in stainless steel columns and are either used at room temperature or are heated to a temperature in the range of 300° C.–450° C. Impurities from gases flowing through a getter column are trapped by the getter materials, thereby providing a purified gas at the outlet of the getter column. For the most part, the gettering process is not reversible, resulting in the eventual saturation of the gas purifier material with the impurities. Therefore, getter-type materials have a finite “life-time,” making the getter columns a “consumable” item. The getter columns contain substantial quantities of expensive getter material and have a finite useful lifetime. The actual period of time that the getter column lasts depends upon the type and amount of impurities, the flow rate of the gas, the duration and frequency of the gas flow, and a number of environmental factors.
Another type of gas purifier system utilizes catalyst-based purifiers that operate by a physisorption principle. These purifiers may be regenerated by thermal or chemical treatment once exhausted.
Current gas purification systems typically comprise at least two purifier vessels for continuous operation, in addition to heating units, switching manifolds, and electronics modules. During purification, one purifier vessel is on-line for purifying a bulk gas. Before this purifier vessel becomes saturated with contaminants, the second purifier vessel is put on-line and the first purifier vessel is regenerated. One method of performing the regeneration process is by heating the purifier vessel and flowing a reactivation gas through the spent purifier material. After regeneration, the purifier bed is cooled and purged and is then ready to purify the bulk gas. Typical systems use hydrogen and/or nitrogen for the regeneration process, each requiring regeneration process gas connections and vent lines.